In the field of petroleum well drilling and logging, electromagnetic logging tools are frequently used to provide an indication of the electrical resistivity of rock formations surrounding an earth borehole. Such information regarding resistivity is useful in ascertaining the presence or absence of hydrocarbons. A typical electromagnetic logging tool includes a transmitter antenna and a pair of receiver antennas located at different distances from the transmitter antenna along the axis of the tool. The transmitter antenna is used to create electromagnetic fields in the surrounding formation. In turn, the electromagnetic fields in the formation induce an electrical voltage in each receiver antenna. Due to geometric spreading and absorption by the surrounding earth formation, the induced voltages in the two receiving antennas have different phases and amplitudes. Experiments have shown that the phase difference (Φ) and amplitude ratio (attenuation, A) of the induced voltages in the receiver antennas are indicative of the resistivity of the formation.
The frequency-domain based operation of these tools imposes certain limitations when such tools are adapted for use in geosteering, where long detection ranges and accurate images are desired. To a limited extent, these features can be provided with wide antenna spacings, multiple measurement frequencies, and complex inversion algorithms.
There have been proposed a number of tools with time-domain based operations, including U.S. Pat. No. 4,297,699, “Radar drill guidance system” by Fowler; U.S. Pat. No. 5,115,198, “Pulsed electromagnetic dipmeter method and apparatus employing coils with finite spacing” by Gianzero; U.S. Pat. No. 6,712,140, “Borehole radar antenna and algorithm, method and apparatus for subsurface surveys” by Van Oers; U.S. Pat. No. 6,778,127, “Drillstring radar” by Stolarczyk; U.S. Pat. No. 7,123,016, “Systems and methods useful for detecting presence and/or location of materials” by Larsen; and U.S. Pat. No. 7,046,009, “Method for measuring transient electromagnetic components to perform deep geosteering while drilling” by Itskovich. It does not appear that any of these proposed tools have proven commercially feasible. The authors believe that this outcome is at least partly the result of costly electronics which prove too fragile for the extreme conditions found in the drilling environment, and partly due to the use of antenna configurations that complicate the inversion and interpretation process.
By way of example, specific system and method embodiments are shown in the drawings and are described in detail below. It should be understood, however, that the drawings and detailed description are not intended to limit the disclosure, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.